The present invention is based on a fuel injection system.
German Published Patent Application No. 198 27 219 describes a fuel injection system for an internal combustion engine having an injector with a fuel jet adjustment plate, which is furnished with first nozzle orifices that are arranged in a first circle, and second nozzle orifices that are arranged in a second circle. The second circle has a diameter that is greater than that of the first circle. The circles are arranged coaxially to a center line of the adjustment plate. Each orifice axis of the second nozzle orifices forms an acute angle with a reference plane, which is perpendicular to the center line of the valve body. The angle is smaller than the angle that is formed between the reference plane and each orifice axis of the first nozzle orifices. In that way, atomized fuel that is injected through first nozzle orifices may be directed away from the atomized fuel that is injected through second nozzle orifices. One result of this arrangement is that the atomized fuel that is injected through the first nozzle orifices does not interfere with the atomized fuel that is injected through the second nozzle orifices, which makes it possible to atomize the injected fuel suitably.
German Published Patent No. 196 42 653 describes a method of forming an ignitable fuel-air mixture. An ignitable fuel-air mixture is formable in the cylinders of direct-injection internal combustion engines, in that fuel is injected into each combustion chamber delimited by a piston, by way of an injector on opening of a nozzle orifice due to a valve element being lifted up from a valve seat surrounding the nozzle orifice. To permit formation of an internal mixture optimized for fuel consumption and emissions at each operating point of the entire engine characteristics map under all operating conditions of the internal combustion engine, in particular in stratified charge operation, the opening stroke of the valve element and the injection time are adjustable.
German Published Patent Application No. 198 04 463 describes a fuel injection system for internal combustion engines having spark ignition of a compressed fuel mixture; this fuel injection system is provided with an injector which injects fuel into a combustion chamber formed by a piston/cylinder arrangement and is equipped with a spark plug projecting into the combustion chamber. The injector is provided with at least one row of injection orifices distributed over the circumference of the injection nozzle. Through controlled injection of fuel through the injection orifices, a jet-guided combustion method is implemented by the formation of a mixture cloud, at least one jet being directed for ignition in the direction of the spark plug. Other jets are provided to assure that the mixture cloud formed is at least approximately closed or contiguous.
The methods of mixture formation and fuel injection systems described in the documents cited particularly share the disadvantages of inadequate homogeneity of the mixture cloud, and the problem of transporting the ignitable mixture to the region of the spark plug""s air gap. In order to enable low-emission, low-consumption combustion, complex combustion chamber geometries, helical valves or swirl mechanisms must be used in such cases to ensure that the combustion chamber is filled with a fuel-air mixture, and that the ignitable mixture is brought into the proximity of the spark plug.
The fuel is sprayed directly onto the spark plug. This results in heavy sooting of the spark plug and frequent thermal shocks, which shorten the service life of the spark plug.
The disadvantage of the method described in German Published Patent No. 196 42 653 for the formation of an ignitable fuel-air mixture is the additional impossibility of accurately dosing small quantities of fuel, particularly in stratified charge operation, because the opening and closing times of the fuel injector cannot be controlled with sufficient precision.
It is furthermore disadvantageous that complicated combustion chamber geometries and fuel injectors with swirl arrangement are difficult and expensive to produce.
The fuel injection system according to the present invention provides at least one row of injection orifices and a central injection orifice, which provide a mixture cloud having a stoichiometric or slightly rich core (0.8  less than xcex less than 1.1) and a lean outer region (1.5  less than xcex less than 2.5) in the combustion chamber.
If the core of the mixture cloud is located in the area of the spark plug air gap at the moment of ignition, the core may ignite quickly, combust completely, and in turn ignite the lean outer region. This combustion method features particularly low nitrogen oxide emissions, since only a small portion of the mixture cloud, i.e., the interior stoichiometric core, burns at a high temperature so that only small quantities of nitrogen oxides are released, whereas the lean outer region, which occupies a significantly greater volume may combust at a lower temperature with negligible nitrogen oxide emissions.
The present invention provides that injection orifices in at least one row of circumferentially arranged injection orifices and the central injection orifice have different diameters. By appropriate selection of the orifice diameter it is possible to create an injection cone having a more or less rich core.
If the diameter of the central injection orifice is selected to be greater than that of the other injection orifices, it is also possible to create a parachute-type shape to the conical injection jet, thus filling the combustion chamber better and also ensuring improved stoichiometric distribution in the mixture cloud.
This may be achieved with two rows of circumferentially arranged injection orifices, if the diameters of the outer row of injection orifices are smallest, the injection orifices of the inner row are somewhat larger, and the central injection orifice has the largest diameter.
A geometrical arrangement of the injection orifices of the inner and outer rows is also advantageous, according to which two injection orifices in the outer row correspond to one injection orifice in the inner row. The number of injection orifices per row is thus twice the number of injection orifices in the row immediately inferior thereto. In this way, not only the homogeneity but also the conformation and stoichiometry of the mixture cloud may be controlled.